Distillation of crude acrylonitrile



United States Patent DISTILLATION 0F CRUDE ACRYLONITRILE Leslie H. Sutherland and Calvin C. Rolland, New 0rleans, La., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application August 26, 1955, Serial No. 530,898

2 Claims. (Cl. 202-57) This invention relates to the preparation of acrylonitrile. More particularly, it relates to refining crude acrylonitrile. Still more particularly, it relates to steam stripping a dilute aqueous acrylonitrile solution.

In the preparation of acrylonitrile by reacting HCN with acetylene, it is the usual practice to scrub the resultant reactor gas with an aqueous solution to absorb the acrylonitrile content. The resultant dilute acrylonitrile solution so obtained is then concentrated by steam stripping. Overheads from the steam stripping operation, comprising. acrylonitrile and impurities including water, are led to a decanter wherein they stratify into an acrylonitrile layer saturated with water and a water layer saturated with acrylonitrile. The concentrated acrylonitrile layer is then subjected to various physical and/or chemical operations to separate impurities and obtain a purified acrylonitrile product. The water layer is returned as reflux to the steam stripper, while the steam stripper bottoms are recycled to the acrylonitrile absorber as absorber feed.

The described steam stripping operation, however, is subject to certain disadvantages. For instance, when operated at its full capacity, excess water is carried over with the overheads. As a result, the take-off from the decanter soon becomes flooded. To obviate this, it is necessary to restrict the operation of the stripper column to substantially less than full capacity. This, however, creates a new problem if the acrylonitrile absorber is dependent upon full capacity operation of the stream stripper for absorber feed. To eliminate these problems, therefore, and still obtain the original object, it has been necessary to employ two, rather than one, stripper columns.

In accordance with this invention, it has now been found that the above-described disadvantages can be overcome while at the same time substantially doubling the capacity of the steam stripper, by having present in the aqueous acrylonitrile solution during the steam stripping operation a small amount of tri-2-ethylhexyl phosphate. Exactly why' the presence of this compound should exhibit such a surprising effect on the steam stripping is not clearly understood. Nor is there any desire to limit the invention by any theory of operation. Whatever the reason, however, the presence of the compound permits operation of the stripping column at full capacity with little, if any, carry-over with the overheads of excess water.

The amount of tri-Z-ethylhexyl phosphate employed may vary over a wide range. .It has been found that as little as 1.5 parts per million of acrylonitrile solution being treated is elfective. Larger amounts are still more effective, but it has been found that employing more than about 20 parts per million shows little added advantage. Usually, it will be found that from about 2l5 parts per million of tri-Z-ethylhexyl phosphate will be adequately effective.

The tri-Z-ethylhexyl phosphate may be introduced at any of various times in the operation. Thus, when the stripper bottoms, which will normally have present some tri-Z-ethylhexyl phosphate, are employed as absorber feed, any additionally required tri-Z-ethylhexyl phosphate may be added directly thereto. It may also be added at any other point prior to steam stripping or even during steam stripping itself. A particularly convenient place for adding the material is in the acrylonitrile-saturated water layer taken from the decanter and employed as a reflux.

The process of this invention is further described by the following examples. These examples are illustrative only and not by way of limitation. All parts are by weight unless otherwise noted.

Example 1 Example 2 The procedure of Example 1 is repeated using the same fractionating column except that 10 parts per hour of a 10% aqueous tri-Z-ethylhexyl phosphate solution is added. Under fl1ese conditions the feed rate of the 3% aqueous acrylonitrile solution can be increased to more than 1800 parts per minute and a clean separation of acrylonitrile product still obtained. The amount of additive in the feed is about 10 p. p. in.

Example 3 Example 2 is repeated except that the amount of tri- 2-ethylhexyl phosphate added is reduced to 1.7 p. p. m. An excellent separation of acrylonitrile product is again obtained when using a feed rate of 1800 parts per minute.

What is claimed is:

1. In refining crude acrylonitrile in which a dilute aqueous acrylonitrile solution is concentrated by steam stripping, the improvement in combination therewith which comprises: having present in said solution during steam stripping a small amount of tri-2-ethylhexyl phosphate.

2. A process according to claim 1 in which the amount of tri-Z-ethylhexyl phosphate is from about 15-20 p. p. In.

No references cited. 

1. IN REFINING CRUDE ACRYLONITRILE IN WHICH A DILUTE AQUEOUS ACRYLONITRILE SOLUTION IS CONCENTRATED BY STEAM STRIPPING, THE IMPROVEMENT IN COMBINATION THEREWITH WHICH COMPRISES: HAVING PRESENT IN SAID SOLUTION DURING STEAM STRIPPING A SMALL AMOUNT OF TRI-2-ETHYLHEXYL PHOSPHATE. 